In recent years, along with a tendency to high brightness and whitening of a light-emitting element such as a light-emitting diode (LED) element, a light-emitting device employing a light-emitting element has been used for illumination, backlights of various displays or large-sized liquid crystal TVs, etc. The substrate for mounting a light-emitting element, to mount a light-emitting element, is usually required to have a high reflectivity to efficiently reflect light emitted from the element.
Accordingly, it has been heretofore attempted to provide a reflection layer on the substrate surface for the purpose of reflecting light emitted from a light-emitting element to a forward direction as far as possible. As such a reflection layer, a reflection layer containing silver (hereinafter sometimes referred to as a silver reflection layer) having a high reflectance is employed.
However, silver is likely to be corroded, and if it is left to stand, a compound such as Ag2S is likely to be formed, whereby the optical reflectance tends to deteriorate. To prevent such a problem, a method has been proposed wherein the surface of silver is coated with a resin such as a silicon resin, an acrylic resin, an epoxy resin or an urethane resin (Patent Document 1). However, by such a method, moisture or a corrosive gas is likely to enter into the resin or from the interface between the silver reflection layer and the resin, whereby the silver reflection layer is corroded as the time passes, and thus it has been difficult to employ such a method for products which are required to have a reliability for a long period of time.
Accordingly, in order to prevent corrosion of a silver reflection layer, a method has been proposed to coat the surface of silver with a glass layer (Patent Document 2). However, the glass layer formed by this method undergoes a color change from yellow to brown by a reaction with silver during the firing, whereby the reflectance tends to deteriorate, and thus, such a method has been inadequate for application to products.
On the other hand, as a method to increase the reflectance without using a silver reflection layer, it has been proposed to use an alumina material or the like having a high reflectance, but firing at a high temperature exceeding 1,000° C. is required, and thus there has been a problem that the load on the production process is substantial.